Wood preservatives serve for protecting wood from damages by vermin or injurious insects such as termite, and quaternary ammonium salts having a high insect- or ant-proof capability and a high antiseptic or mildew-proof capability are known as the wood preservatives.
The quaternary ammonium salts are usually produced by quaternarizing tertiary amines. However, in many cases, since alkyl halides such as methyl chloride and methyl bromide are used as a quaternarizing agent, halogen ions such as chlorine ion and bromine ion tend to be inevitably included in the obtained quaternary ammonium salts. As a result, when the halogens contained in the quaternary ammonium salts are contacted with metals, rusts tend to be generated, thereby causing problems such as damage to production apparatuses and, therefore, poor production efficiency. To solve the problems, organic acids have been used instead of the alkyl halides.
As the method for producing the quaternary ammonium salts using the organic acids, there are known an ion exchange resin method and an electrodialysis. However, in the ion exchange resin method, the ion-exchanged resin used must be subjected to an alkali treatment or an acid treatment for regenerating the resin, or the ion exchange resin must be periodically replaced with new one, resulting in prolonged production process, high costs for facilities and high production costs. Also, since the reaction used in the ion exchange resin method cannot be conducted at a high concentration, the ion exchange resin method is unsuitable for mass production of the quaternary ammonium salts. On the other hand, in the electrodialysis, only limited solvents are usable therein because of possibility of explosion upon using inflammable solvents, resulting in poor flexibility of the method.
Further, JP 2000-336070A discloses a method for synthesizing a quaternary ammonium salt by reacting a tertiary amine with an alkyl halide and then adding an alcohol containing sodium, etc., and an organic acid to the obtained reaction product for replacement of a counter ion of the salt. However, in this method, since a large amount of the solvent must be used in the reaction, the productivity tends to be lowered, and additional operations for removal or recovery of the solvent are also required. For this reason, in the above method, there tend to occur problems such as increase in size of production facilities, complicated production process and high production costs.
JP 4-57661B, JP 6-96243B and FUJIMOTO, Takehiko, “NEW INTRODUCTION TO SURFACE ACTIVE AGENTS”, Sanyo Chemical Industries, Ltd., 1st Edition, Oct. 1, 1981, p. 68, disclose a method for producing a quaternary ammonium salt from a tertiary amine and an alkyleneoxide.
However, these conventional methods have problems such as low reaction rate, poor hue of the resultant products, deterioration in pH or hue upon storage.